ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus PublicationsGöttingen, Germany10.5194/acp-8-2493-2008Hourly resolved cloud modification factors in the ultravioletStaigerH.17den OuterP. N.2BaisA. F.3FeisterU.4JohnsenB.5VuilleumierL.61German Meteorological Service, Freiburg, Germany2National Institute for Public Health and the Environment, Bilthoven, The Netherlands3Aristotle University, Thessaloniki, Greece4German Meteorological Service, Lindenberg, Germany5Norwegian Radiation Protection Authority, Oesteraas, Norway6Federal Office of Meteorology and Climatology MeteoSwiss, Payerne, Switzerland7emeritus090520088924932508This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from http://www.atmos-chem-phys.net/8/2493/2008/acp-8-2493-2008.htmlThe full text article is available as a PDF file from http://www.atmos-chem-phys.net/8/2493/2008/acp-8-2493-2008.pdf

Cloud impacts on the transfer of ultraviolet (UV) radiation through the
atmosphere can be assessed by using a cloud modification factor (CMF). CMF,
which is based on total global solar irradiation (SOL<sub>CMF</sub>), has proved to
be a solid basis to derive CMFs for the UV radiation (UV<sub>CMF</sub>). This is an
advantage, because total global irradiance, the basis for SOL<sub>CMF</sub>, is
frequently measured and forecasted by numerical weather prediction systems
and includes all relevant effects for radiation transmission, such as cloud
optical depth, different cloud layers, multiple reflection, as well as the
distinct difference as to whether the solar disc is obscured by clouds or
not. In the UV range clouds decrease the irradiance to a lesser extent than
in the visible and infrared spectral range. Thus the relationship between
CMFs for solar radiation and for UV-radiation is not straight forward, but
will depend on whether, for example, the solar zenith angle (SZA) and
wavelength band or action spectrum in the UV have been taken into
consideration. Den Outer et al. provide a UV<sub>CMF</sub> algorithm on a daily
basis, which accounts for these influences. It requires as input a daily
SOL<sub>CMF</sub> and the SZA at noon. The calculation of SOL<sub>CMF</sub> uses the
clear-sky algorithm of the European Solar Radiation Atlas to account for
varying turbidity impacts. The algorithm's capability to derive hourly
UV<sub>CMFs</sub> based on the SZA at the corresponding hour and its worldwide
applicability is validated for erythemal UV using observational data
retrieved from the databases of the COST-Action 726 on "Long-term changes
and climatology of UV radiation over Europe" and the USDA UV-B
Monitoring Program. The clear-sky part of the models has proved to be of
good quality. Accumulated to daily doses it forms a tight cluster of points
to the highest measured daily sums. All sky model performances for hourly
resolution are shown to be comparable in accuracy with the well performing
daily models of the COST-726 model intercomparison.